Thank in advance for any help.
 
I sometimes play around with midi files that other people make and in some cases they use full chords in rock songs that require power chords because the guitar tracks are played back with distortion.  I have tried to manually convert the full chords but it is a painstaking and lengthy process.  I am not a guitarist so "just playing that part" is not something I'd even entertain.  Does anyone know of a way to take a midi track and instantly convert the full chords to power chords?  Is there a VST or even a CAL script that will do this?

The usual power chord is usually just the root and the fifth.
 
So if you've got a full Amajor chord, for example, delete everything but the lowest A and the E above it.  I'd be surprised if there was an automated way to do this.

Im not sure of a Cal script either, But what I did was make a Midi Chord library.
Power Chords, Arp Chords, minors, Majors, ext.
Slug is correct. in the Power chord it is simply the root, the 5th and the octave.
I happen to find some chords like E Power chord sounds better by adding the high b and high e along with the normal root, 5th and octave. This gives a more realistic feel, like a real open E with high strings ringing.
 
Once you create this library, its easy. Just drag and drop it from the browser on to the time line midi track

I'm afaid your wish is next to impossible. There's no way a robot can determine the chord viocing and only strip away the 3rd, 4t etc. The 4TH of a G chord is the root of a C chord so how on earth will that work. 
You need to play your own parts that way you want them ( the easy way) or suck it up and spand a lot of time editing. 

Cactus Music
I'm afaid your wish is next to impossible. There's no way a robot can determine the chord viocing and only strip away the 3rd, 4t etc. The 4TH of a G chord is the root of a C chord so how on earth will that work. 
You need to play your own parts that way you want them ( the easy way) or suck it up and spand a lot of time editing. 

I agree that it's next to impossible to automate, but it shoudn't take a lot of time to fix.  Select the cursor as an eraser, and click away on any notes not wanted in PRV.  I'd imagine you could do a standard rock/pop song in 5-10 minutes.

 "I'd imagine you could do a standard rock/pop song in 5-10 minutes."  
Optimistic timeline :) It would take me a lot longer, and I guess this all depends on the OP's knowledge of chord theory. 
 
A lot would depend on the complexity of the arrangement, A  3 chord song is way more possible than a 6 chord song. 
Example 3 chords G-C-D. 
G chord G B D   so B has to go
C chord C E G    so E has to go
D chord D F# A  so F# has to go   
So we are keeping G, D,  C and  A  
So you could do it in a couple of clicks using the Piano keys on the Left to highlight all the B, E and F#. at once. 
But then adding any other chords to the arrangement will probably require those notes again so now your back to editing chord by chord. 
 
And visually one would hope to just delete the middle note, but many of these are using inversions so you have to be careful.  
 
I think what the OP is dealing with are most free midi downloads are done by keyboard players who tend to not approach chords the way a guitar does..Then or worse, generated by Band in a Box type software  and they use overly complex chords which sound real bad when what was required was more fundamental tones.  It's rare that I can find a midi file that is well done. I was even just looking for a few Christmas songs and it blows me away how many bad versions there are of even a basic melody. I always end up rolling my own. It's much faster than polishing the old turdd 

Inversions will also mess with automation options, so the advice of manual replacement/editting would get quicker results than searching for a solution. Root/5th (and often octave, depending) are it, so the eraser tool in the PRV is your best friend. PRV is also nice for identifying intervals because all semitones are the same size visually.

Well thanks for the replies.  I was just wondering if there are chord detectors (which I think I've seen a few) there should be a way to generate a power chord replacement.  It's probably out there somewhere.  I tried lots of searches and came up empty.
 
I understand chord theory ok but my main problem is identifying the chords and then picking the notes to erase.  Like I said, I have done it manually for quite a while but it's a serious pain.

What program are you using to convert the audio to midi, do you have Meloodyne Studio?
 
When you have the midi track, you can add the Midi FX to the FX bin. There is a chord analyzer in there. It will often suggest inversions. So will help identify the root and 5th.
 
Another approach would be to you a chorder, then you could just play single notes from a keyboard and have it produce the 1st 5th and 8th for you.